1. Technical Field
The present invention relates to a method for metallization of a glass element, including an optical waveguide such as an optical fiber, a large diameter waveguide cane structure or planar waveguide.
2. Description of Related Art
There are known methods for metallization of optical fibers and large glass structures. These methods include vapor deposition methods, wet chemical processes, and additional methods that utilize a combination of the two approaches. The inherent problems with vapor deposition methods are heat-induced damage to the fiber substrate, poor adhesion of the metal coating, and non-uniformity of the coating. Wet chemical processes are currently the most attractive metallization methods for the highest coating adhesion and quality.
Standard wet chemical processes for the metallization of large glass substrates have been tried to metallize optical fiber. These known wet chemical processes utilize tin chloride (SnCl2) for surface sensitization and palladium chloride (PdCl2) for activation, respectively, prior to metallization by electroless nickel deposition. Common problems with using these known wet chemical processes to metallize optical components lie in the occurrence of inconsistent results and the production of incomplete coatings.
Another approach for the metallization of an optical fiber disclosed by Filas et al. (U.S. Pat. No. 5,380,559) uses tin fluoride (SnF2) in place of tin chloride for sensitization. The primary difficulties of this approach are caused by the unstable nature of tin fluoride. Subsequently, preparation of the tin fluoride solution, sensitization, and activation must be carried out in a non-oxygen inert atmosphere (such as nitrogen or argon) in order to prevent oxidation of the sensitized fiber and tin fluoride solution.
Further, it is known to complete the metallization of an optical fiber using an electroless metallic solution consisting of the Fidelity solution 4865A (constituent part which is nickel sulfate), the Fidelity solution 4865B (constituent parts which are sodium hypophosphite, sodium hydroxide and acetic acid) and de-ionized water in a ratio of 1:3:16. However, this known electroless metallic solution is not active enough with such small surface areas and did not plate when using tin chloride (SnCl2). This standard mix resulted in the electroless metallic solution having a sodium hypophosphite concentration of 30 g/L. This standard mix contained too many stabilizers, which were preventing the plating from initializing on the fiber.
In view of this, it was found that, when plating optical fibers in the known electroless nickel solution, the plating rates and platability of the optical fibers were inconsistent and unacceptable.
The reasons for this include the fact that:
1) The optical fiber has a small surface area, i.e. a small load, in relation to known stabilizers. The electroless nickel solution is autocatalytic. The larger the area to be plated, the easier it is for the reaction to start and maintain itself. An optical waveguide, such as an optical fiber, is not a xe2x80x9cloadxe2x80x9d that lends itself to an easy start.
2) The commercially available electroless nickel solutions, such as the aforementioned Fidelity solutions, contain too many stabilizers. These chemicals were originally intended to prevent the bath from xe2x80x9cplating outxe2x80x9d (which is feeding off of itself at an uncontrolled rate until all the nickel is gone), and to prevent plating on the sides of its container. These stabilizers interfere with the plating on the optical fiber.
In view of the aforementioned, there is a need in the industry for a better way to metallize an optical waveguide using a wet chemical process.
In its broadest sense, the present invention provides a method of applying a metal coating to a glass element having a small surface area comprising a unique set of steps. The glass element may include an optical waveguide such as an optical fiber typically having a diameter of 125 microns, a large diameter waveguide cane structure having a diameter of at least 0.3 millimeters or greater, or a planar waveguide.
First, the stabilizers of an electroless metallic solution are partially depleted therein. One way of partially depleting the stabilizers is to provide an electroless metallic solution having a concentration of sodium hypophoshite of about 25 grams/liter.
The electroless metallic solution may comprise a first solution having nickel sulfate (such as the Fidelity solution 4865A), a second solution having sodium hypophosphite, sodium hydroxide and acetic acid (such as the Fidelity solution 4865B) and de-ionized water in a ratio of about 1:1:18; and sodium hypophosphite crystals, which are added to create a sodium hypophoshite concentration of about 25 grams per liter in the electroless metallic solution. In effect, the volume of the Fidelity solution 4865B is reduced and the de-ionized water is increased of the known electroless metallic solution (i.e., 1:3:16 solution), as described hereinbefore, to achieve the desired partial depletion of the stabilizers in the electroless metallic solution.
Alternatively, another way of partially depleting the stabilizers is to place a dummy load into an electroless metallic solution. The dummy load may be a rectangular block of metal, which may be formed of a low carbon steel and may include a threaded cylindrical passage therein.
The electroless metallic solution used with the dummy load may comprise a first solution having nickel sulfate (such as the Fidelity solution 4865A), a second solution having sodium hypophoshite, sodium hydroxide and acetic acid (such as the Fidelity solution 4865B) and de-ionized water in a ratio of about 1:3:16.
After partially depleting the stabilizer of the electroless metallic solution, the glass element is immersed in the electroless metallic solution to deposit a metal coating, for example, to the glass element, for a predetermined length of time depending on a desired deposition thickness. The nickel coating may also be further coated with gold to form a gold on nickel coating. The gold coating prevents the nickel coating from oxidizing.
The method also includes other steps such as initialization, sensitization and activation of the optical waveguide.
One advantage of the method of the present invention is that it can be executed in a tabletop environment with no need for an inert atmosphere.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.